The known sewing machines are provided with various means for feeding an upper thread to the sewing needle, and other means, including a bobbin, for feeding the lower thread to the sewing needle. Monitoring the condition, particularly breakage, of the upper thread is relatively simple, and several methods are in use today, as described for example in U.S. Pat. No. 3,843,883. However, monitoring the condition of the lower bobbin thread is somewhat more problematical, and although a number of systems have been devised for doing this, the known systems are still not entirely satisfactory. The main reason for this is because of the complexity of the path of the lower thread out of the bobbin, which enables very little room for detection. Various methods for detecting an empty bobbin condition using a light beam are known, for example as described in U.S. Pat. Nos. 4,237,807 and 4,212,257, and in British Patents 1,335,677 and 2,078,798. However, these known techniques are usually of complicated construction and generally do not detect all the conditions of the bobbin thread, including thread exhaustion and thread breakage.
Our prior U.S. Pat. No. 4,934,292 disloses an arrangement for detecting when a bobbin stops rotating, e.g., because of an empty bobbin. The detection of this condition may be used for terminating the operation of the sewing machine to enable refilling or replacement of the empty bobbin and to continue with the stitching operations.
Those specifications relate to a conventional bobbin on which the thread is wound. Recently, however, "pre-wound bobbins" have gained popularity. These are coils of thread without a bobbin, held together by a weak glue so as not to interfere with the release of the thread. By leaving out the bobbin, the need to rewind the thread on the sewing head is eliminated, thus saving time and trouble. This bobbin-less coil of thread, however, cannot be used with the detecting arrangements disclosed in the above patent specifications.
Another problem with sewing machines arises when a stitching operation is terminated in the middle because of an empty bobbin and is then resumed with a new bobbin. This results in a break in the stitch and is particularly unsightly in "top stitchings", such as collars, cuffs, pockets, etc., viewable from the outside of the garment, as distinguished from "inside stitches" which are not viewable from the outside of the garment.
Our prior U.S. Pat. No. 4,934,292 discloses a bobbin structure which permits the bobbin, even when it stops rotating, to nevertheless supply a sufficient length of thread in order to finish a stitching operation. However, this structure is also incompatible with the use of a bobbin-less coil of thread.